The L-carrier system was one of a series of carrier systems developed by AT&T for high-capacity transmission for long-distance communications. Over a period from the late 1930s to the 1970s, the system evolved in six significant phases of development, designated by Bell System engineers as L-1 through L-5, and L-5E. Coaxial cable was the principal transmission medium in all stages, initially lending the system another description i.e. the coaxial system. It was the successor to a series of previous carrier systems, typically identified by capital letters. In the 1960s, the system was hardened against the dangers of the Cold War using complete placement of all terminal and repeater equipment in hardened underground vaults.

Initial development and testing of the coaxial system took place between 1935 and 1937 on a test bed of a 95-mile (153 km) two-way coaxial cable between locations in New York City and Philadelphia. A distance of 3,800 miles (6,100 km) was simulated by repeatedly remodulating signals and looping them twenty times between the endpoints. The system provided 240 channels over a single circuit.

The first production installation of the L-1 carrier system went into service between Stevens Point (WI) and Minneapolis (MN) in 1941 over a distance of almost 200 miles (320 km). with a capacity of 480 channels, far more than could be carried by balanced pair carrier systems, and cheaper per channel for high-usage routes.

A small-scale L-type carrier system between Baltimore (MD) and Washington, D.C. was intended for short-distance low-volume traffic. The system likely to be designated L-2 was abandoned at an early stage in the 1940s.

With the anticipation of the end of war-time responsibilities, AT&T announced in December 1944 a development plan for nationwide build-out of the coaxial carrier network for support of not only long-distance telephone service, but also for television transmissions. The result of post-war research of this goal was the definition of the L-3 carrier system.

Each successive version had at least twice as many channels as the previous version, culminating in the L-5E design in 1976. AT&T Long Lines built two coast-to-coast systems of L-3 as well as shorter ones connecting major cities, especially the big cities of the eastern United States, as a supplement to the mainstay microwave radio relay systems. Some were later upgraded to L-4, while others were simply overbuilt with a new L-5 system.